Combinations. The purpose of the study is to try to understand one of the mechanisms by which NK cells recognize and kill some virally infected cells, tumor cells, and allogeneic cells. Healthy human donors are used as the source of fresh peripheral blood mononuclear cells containing NK cells. The targets are generated using the human HLA class I negative Iymphoblastoid cell line 721.221,transfected with the gene encoding either the HLA-B7 wild type molecular, or HLA-B7 with a single point substitution in or around the peptide binding groove. Twenty-seven variant-expressing cell lines have been tested as targets for NK cells in cytolytic assays,namely 51CR-release assays. Additional variants are being tested. Expression of the HLA-B7 on the cell surfaces is monitored by fluorescence activated cell sorter analysis using HLA class I-and HLA-B7-specific monoclonal antibodies. HLA-B7 variant-expressing target cells exhibit significant differences in their susceptibility to NK killing. Seven of 11 peptide binding groove residue mutations increase target cell susceptibility to NK killing by 40-60% as compared to the wild type NLA-B7-expressing target, suggesting the importance of HLA class I-peptide combinations in NK function. Some NK cell lines have been tested in cytolytic assays, and the results support the findings obtained with bulk peripheral blood mononuclear cells.